The present invention generally relates to methods and apparatus for controlling the registration of one web, tape or strand of continuous plastic material relative to another web, tape or strand of elongated continuous structure, both of which are being fed to a packaging machine. In particular, the invention relates to methods and apparatus for controlling the registration of a continuous zipper material relative to a continuous web in a thermoforming packaging machine.
In cases where a continuous zipper without pre-sealing and without sliders must be joined with a continuous web of packaging film having thermoformed pockets, there is a need for the zipper to be properly aligned with the web of film (i.e., straightness and cross-machine alignment), but there is no need to register the zipper relative to the web in a machine direction. This is due to the fact that the zipper has a constant profile along its length and thus has no structural features that need to be registered relative to the pockets thermoformed on the web of packaging film.
The continuous zipper material typically comprises a pair of continuous zipper strips, each zipper strip having a respective constant profile produced by extrusion. Typically, the respective zipper strip profiles have complementary shapes that allow the zipper strips to be interlocked. These closure profiles may be of the rib-and-groove variety, the interlocking-hook variety or any other suitable fastenable structures. Pre-sealing of the zipper material involves crushing and fusing the zipper strips at spaced intervals along the zipper at locations where the zipper material will be ultimately cut when each finished package is severed from work in process. In cases where the zipper material is pre-sealed before entering the packaging machine, it is important that the pre-seals be properly registered relative to the pockets thermoformed on the web of packaging film.
In cases where sliders are inserted at spaced intervals along the zipper before the latter enters the packaging machine, it is common to combine the joinder of the zipper strips at spaced intervals with the formation of slider end stop structures on the zipper. Although slider end stops can be placed on or inserted in the zipper, it is common practice to simply deform and fuse the thermoplastic material of the zipper strips wherever slider end stops are needed. Typically, the zipper material is deformed by application of ultrasonic wave energy and shaping the thus-softened zipper material to form a slider end stop structure. Typically the slider end stop structure forms back-to-back slider end stops when bisected. The slider end stop structure is formed at a location such that its midplane will be coplanar with the plane of cutting when the finished package is severed from the work in process. Thus, it is important that the slider end stop formations on the zipper be properly registered relative to the pockets thermoformed on the web of packaging film.
There is a need for a simple, inexpensive and accurate scheme for controlling the registration of one elongated continuous structure (e.g., plastic zipper), with attachments (e.g., sliders) or formed features (e.g., slider end stop structures), as it is fed to a sealing station, where it is joined to and later pulled by another elongated continuous structure (e.g., a web of packaging film), with formed features (e.g., thermoformed pockets or troughs). The registration control equipment should also be easy to install. Also, the scheme for controlling registration of the pulled elongated continuous structure relative to the pulling elongated continuous structure should be adaptable to machines in which each advance of the latter is equal in distance to a single unit or package length or multiple unit or package lengths.